The present invention relates to a semiconductor device, and more particularly to a gate controlled semiconductor device which is turned on and off by applying a gate control signal thereto.
A gate control semiconductor device such as, for example, a gate turn-off thyristor is constructed similar to a usual reverse-blocking three-terminal thyristor. The gate turn-off thyristor is transmitted from a conductive state to a non-conductive state by supplying the current to a gate electrode thereof, and thereby the main current flows into the gate turn-off thyristor. In order to change the thyristor to the non-conductive state from the conductive state, the main current must be made zero during some predetermined interval by some external means. The gate turn-off thyristor may be returned to the non-conductive state from the conductive state by supplying the negative current, which is the current flowing in the reverse direction for reverse-biasing a cathode junction. A ratio of a value of the reverse current and that of the main current (a load current) which is turned off by the reverse current, is called a turned-off gain (g=Ica/Igr), where Ica is the main current of the turn-off thyristor, and Igr is the reverse current. In order to increase the turn-off gain of the gate turn-off thyristor, it is necessary to make the base width larger and/or to make a gold diffusion condition stronger. The more important thing is the configuration of a cathode electrode which is required to make a transverse resistance between a cathode electrode and a gate electrode, as small as possible, in order to sweep effectively the current of the cathode.
In the conventional gate turn-off thyristor, the current mu-factor is restricted to lower values than that of the usual thyristor in order to increase the current interrupting capability due to the gate electrode. Furthermore, the gate turn-off thyristor is designed in shape so that the cathode electrode is located as near the gate electrode as possible, in order to eliminate the inner impedance between the cathode electrode and the gate electrode. Accordingly, the cathode electrode is divided into many small pieces and, as a result, the length of opposing faces of the cathode electrode and the gate electrode become inevitably long and, in particular, the cathode junction of a large capacity gate turn off thyristor is divided into a larger number of small pieces. Accordingly, in a gate turn-off thyristor constructed above, the gating current is more than ten times and/or even one hundred times that of the usual power thyristor.
In general, the gate turn off thyristor is often used to an inverter apparatus and a chopper apparatus, when a motor is used as a load of the apparatus employing the gate turn-off thyristor, the gating current must be supplied continuously to the gate electrode of the gate turn-off thyristor for both a conductive interval and a nonconductive interval. This requision is more serious in the gate turn-off thyristor than the usual thyristor. In the gate turn-off thyristor, the main current is interrupted due to the change of the load current even when the gate turn-off thyristor has been turned on, and then holds the current by a part of the cathode assembly. In this case, a long time interval is required to expand the conduction area since the cathode electrode is formed by the many pieces, therefore the current density becomes large in a part of the cathode electrode. Under these conditions, when the gate turn-off thyristor is turned off, the off operation of the gate turn-off thyristor is decreased, while the gate turn-off thyristor is destroyed permanently. Particularly, in the gate turn-off thyristor having (many) separated emitter-cathode, only. A part of the cathode holds the current and the conductive area does not expand to other separated emitters, and as a result the gate turn-off thyristor is destroyed.
In this manner, the gate turn-off thyristor requires not only many times the gating current, than the usual current in firing, but another requirement is to supply the large gating current for the conduction interval. On the other hand, to improve the gating sensitivity a thyristor having an amplificating function is proposed. The amplificating function, however, operates only when the thyristor is fired. Consequently, it is strongly requested to improve the gating sensitivity of the gate turn-off thyristor.